The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel,
which discharge vessel encloses a discharge space containing a filling of mercury and an inert gas in a gastight manner,
electrodes being arranged in the discharge space for generating and maintaining a discharge in said discharge space,
and an electrode shield at least substantially surrounding at least one of the electrodes.
In mercury-vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inner surface of the discharge vessel may be provided with a luminescent layer containing a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, for example to Uv-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation. Such discharge lamps are therefore also referred to as fluorescent lamps.
A low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph is known from DE-A 1 060 991. In said known lamp, the electrode shield surrounding the electrode is made from thin sheet titanium. By using an electrode shield, which is also referred to as anode shield or cathode shield, blackening at an inner surface of the discharge vessel is counteracted. In this respect, titanium serves as the getter for chemically binding oxygen, nitrogen and/or carbon.
A drawback of the use of a metal or metal alloy is that it may cause a short-circuit of the electrode wires. In addition, the metals in the electrode shield may amalgamate with the mercury present in the lamp and, thus, absorb mercury. As a result, the known lamp requires a relatively high dose of mercury to obtain a sufficiently long service life. Injudicious processing of the known lamp after its service life has ended adversely affects the environment.
It is an object of the invention to provide a low-pressure mercury-vapor discharge lamp, which has a relatively low mercury consumption.
To achieve this, the electrode shield is made from a ceramic material.
For the proper operation of low-pressure mercury-vapor discharge lamps, the electrodes of such discharge lamps include an (emitter) material having a low so-called work function (reduction of the work function voltage) for supplying electrons to the discharge (cathode function) and receiving electrons from the discharge (anode function). Known materials having a low work function are, for example, barium (Ba), strontium (Sr) and calcium (Ca). It has been observed that, during operation of low-pressure mercury-vapor discharge lamps, material (barium and strontium) of the electrode(s) is subject to volatilization. It has been found that, in general, the emitter material is deposited on the inner surface of the discharge vessel. It has further been found that the above-mentioned Ba (and Sr), which is deposited elsewhere in the discharge vessel, no longer participates in the light-generating process. The deposited (emitter) material further forms mercury-containing amalgams on the inner surface, as a result of which the quantity of mercury available for the discharge operation decreases (gradually), which may adversely affect the service life of the lamp. In order to compensate for such a loss of mercury during the service life of the lamp, a relatively high dose of mercury is necessary, which is undesirable from the point of view of environmental protection. The inventors have recognized that the provision of an electrode shield, which surrounds the electrode(s) and is made from a ceramic material, reduces the reactivity of materials in the electrode shield relative to the mercury present in the discharge vessel, leading to the formation of amalgams (Hgxe2x80x94Ba, Hgxe2x80x94Sr). In addition, the use of an electrically insulating material precludes the development of short circuits in the electrode wires and/or in a number of windings of the electrode(s). The known lamp has an electrode shield of an electroconductive material, which, in addition, relatively readily forms an amalgam with mercury. The mercury consumption of the discharge lamp is limited by substantially reducing the degree to which the material of the shield surrounding the electrode(s) reacts with mercury.
The electrode shield itself should not appreciably absorb mercury. To achieve this, the material of the electrode shield includes at least an oxide of at least one element of the series formed by magnesium, aluminum, titanium, zirconium, yttrium and the rare earths. Preferably, the electrode shield is made from a ceramic material which comprises aluminum oxide. Particularly suitable electrode shields are manufactured from so-called densely sintered Al2O3, also referred to as DGA. An additional advantage of the use of aluminum oxide is that an electrode shield made of such a material is resistant to relatively high temperatures ( greater than 250xc2x0 C.). At such relatively high temperatures, there is an increased risk that the (mechanical) strength of the electrode shield decreases, thus adversely affecting the shape of the electrode shield. If a metal or metal alloy is used as the electrode shield, as is the case in the known discharge lamp, the temperature of the electrode shield must not be too high to prevent that the metal or one of the metals of the metal alloy begins to deform or evaporate, thereby giving rise to undesirable blackening of the inner surface of the discharge vessel. (Emitter) material originating from the electrode(s) and deposited on an electrode shield of aluminum oxide which is at a much higher temperature, cannot or hardly react with the mercury present in the discharge, as result of said high temperature, so that the formation of mercury-containing amalgams is at least substantially precluded. In this manner, the use of an electrode shield in accordance with the invention serves a dual purpose. On the one hand, it is effectively precluded that the material originating from the electrode(s) is deposited on the inner surface of the discharge lamp, and, on the other hand, it is precluded that (emitter) material deposited on the electrode shield forms amalgams with the mercury present in the discharge lamp. Preferably, in operation, the temperature of the electrode shield exceeds 250xc2x0 C. An advantage of such a relatively high temperature is that, in particular, in the initial stage, the electrode shield becomes hotter than in the known lamp, as a result of which any mercury bound to the electrode shield is released more rapidly and more readily.
An additional advantage of the use of a ceramic electrode shield of aluminium oxide, which surrounds the electrode, is achieved in lamps which are operated on a ballast which can be dimmed, for example a so-called high-frequency regulating (HFR) dimming ballast, in which, particularly at dimmed light intensities, excessive evaporation of electrode-emitter material may occur, said electrode generally being additionally heated under said conditions while using a so-called xe2x80x9cbias currentxe2x80x9d. The electrode shield captures this material and effectively precludes the formation of amalgams. As a result, the mercury consumption of the low-pressure mercury-vapor discharge lamp is limited.
The shape of the electrode shield and its position relative to the electrode influence the temperature of the electrode shield. A further embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that the electrode shield is tubular in shape. Electrodes in low-pressure mercury-vapor discharge lamp are generally elongated and cylindrically symmetric, for example a coil with windings about a longitudinal axis. A tubularly shaped electrode shield harmonizes very well with such a shape of the electrode. Preferably, an axis of symmetry of the electrode shield extends substantially parallel to, or substantially coincides with, the longitudinal axis of the electrode. In the latter case, the average distance from an inside of the electrode shield to an external dimension of the electrode is at least substantially constant.
A particularly preferred embodiment of the low-pressure mercury-vapor discharge lamp is characterized in accordance with the invention in that an inner circumference d, of the electrode shield meets the relation:
1.25xc3x97dexe2x89xa6dsxe2x89xa62.5xc3x97de,
where de is an outer circumference of the electrode. In said area, the temperature of the electrode shield during operation of the low-pressure mercury-vapor discharge lamp is so high that it is at least substantially precluded that (emitter) material deposited on the electrode shield by evaporation or so-called sputtering from the electrode(s), reacts with the mercury present in the discharge, so as to form amalgams. The lower limit, 1.25xc3x97de, of the inner circumference ds of the electrode shield ensures that (mechanized) mounting of the electrode shield does not lead to too small an interspace between the electrode shield and the electrode. An inner circumference ds of the electrode shield below 2.5xc3x97de ensures that, in operation, the temperature of the (emitter) material deposited on the electrode shield is in the desired temperature range to effectively counteract the formation of amalgams.
Preferably, the electrode shield is provided with a slit on a side facing the discharge space. A slit in the electrode shield in the direction of the discharge causes a relatively short discharge path between the electrodes of the low-pressure mercury-vapor discharge lamp. This is favorable for a high efficiency of the lamp. The slit preferably extends parallel to the axis of symmetry of the electrode shield (so-called lateral slit in the electrode shield). In the known lamp, the aperture or slit in the electrode shield faces away from the discharge space. In an alternative embodiment, the electrode shield is tubular in shape and not provided with a slit.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.